In general, inkjet printing machines or printers include at least one printhead unit that ejects drops of liquid ink onto recording media or an imaging member for later transfer to media. Different types of ink can be used in inkjet printers. In one type of inkjet printer, phase change inks are used. Phase change inks remain in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The printhead unit ejects molten ink supplied to the printhead onto media or an imaging member. Such printheads can generate temperatures of approximately 110 to 120 degrees Celsius. Once the ejected ink is on media, the ink droplets solidify. The printhead unit ejects ink from a plurality of inkjet nozzles, also known as ejectors.
The media used in both direct-to-paper and offset (transfix) printers can be in web form. In a web printer, a continuous supply of media, provided in the form of a roll, is entrained onto rollers that are driven by motors. The motors and rollers pull the web from the supply roller through the printer to a take-up roller. Rollers are arranged along a linear media path, and the media web moves through the printer along the media path. As the media web passes through a print zone opposite the printhead or heads of the printer, the printheads eject ink onto the web.
Inkjet printers use solid ink or phase change ink, after printing the solidified ejected ink is warmed by a heater to soften or melt the ink on the media. The melted ink is then fixed to the media by a pressurized nip formed by a spreader drum, which includes a hard surface or non-conformable surface, and pressure roller, which includes a compressible surface. An oil, also known a release agent, is deposited on the surface of the spreader drum and is spread by a metering device, typically a urethane metering blade. As the media with softened ink moves through the nip, the oil on the surface of the spreader drum prevents the compressed ink from offsetting to the spreader drum. After the media image has been compressed to fix the image to the media, the media can be directed to finishing equipment which applies a coating varnish, such as a latex based coating, which provides a protective barrier to the deposited ink and which can also provide a selected finish, such as a glossy finish, to the final documents. The finishing equipment also cuts the continuous web into sheets.
Existing continuous web phase change inkjet printing systems combined with in-line coating systems can perform inadequately when an excessive quantity of release agent remains on the surface of an image moving through the pressurized nip. Even though the image moves through the nip for a relatively short period of time, typically a fraction of a second, for instance milliseconds, an excessive quantity of release agent can remain. In some instances, the excessive quantity of release agent is caused by a worn metering blade found in a drum maintenance unit (DMU) of the printer. If the blade is sufficiently worn, the DMU can leave too much release agent on the spreader drum. The worn metering blade thereby supplies too much oil to the surface of the spreader drum and consequently to the printed media/image. This in turn results in poor wettability of the in-line coating solution to the media/image. If the in-line coating is improperly wetted due to an excessive quantity of release agent, the in-line coating, typically a latex coating/varnish, is not spread evenly across the image but instead is spread unevenly such that some areas of the image include little or no release agent and other areas include too much latex coating/varnish. Consequently, the images are less durable than needed, thereby resulting in degraded durability performance. In such systems, the system delivering the release agent to the spreader drum is not sufficiently robust to deliver the required quantity of release agent at the rates and duty cycle demands.
Another failure mode occurs when the quantity of release agent is insufficient to adequately coat the surface of the spreader drum. In such a situation, the final product suffers from an objectionable product failure rate which is caused by ink offsetting to the spreader drum due to inadequate continuous supply of release agent to the spreader drum surface. Under these conditions, the images which appear on the continuous web can be incomplete, uneven, or smudged.
In one known embodiment, the method for monitoring the application of release agent to the spreader drum is to print a specific test target and perform a physical analysis of the printing system based on the test target, to thereby determine the concentration of release agent being applied. This method requires printing a sample of a known image, which is not part of a customer print job. The printed known image must then be removed from the customer workflow and typically sent offsite for analysis, the results of which can often take days. Consequently, customer workflow can be interrupted for an undesirable period of time, especially since there is a low probability that the problem can be identified in time to prevent a printer failure. This is because a failure of the printer can occur within minutes or hours after it is determined that a physical analysis of the printing system should be made to identify a problem. If a problem related to the application of the release agent is not detected prior to or near the onset of the problem, failures can result leading to unacceptable downtime, labor intensive cleaning and/or replacement of damaged components. Consequently, improvements to a printing system and to printing images by taking into account the application of release agent to the spreader drum, the quantity of release agent being deposited on the continuous web, and conditions occurring in the printer during periods of non-printing or non-use are desirable.